The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Multicasting is a technology that is widely spread, especially for video streaming, such as Internet Protocol TeleVision (IPTV).
Multicasting avoids bandwidth waste that would be induced by the aggregation of unicast data flows towards the receivers.
In wireless networks, multicast data packets are generally emitted in a robust and conservative mode by the physical layer because packets shall be received by any subscribers of the multicast data flow, and because packets cannot be retransmitted by a MAC retransmission scheme.
Contrary to unicast data packets, multicast data packets are emitted in Non-Acknowledged mode, which means that the receiver will not send an acknowledgment upon reception of a data packet.
A robust transmission by the physical layer can also be chosen because the transmitter has less information about reception condition in the receivers. Therefore, it is really difficult to implement an efficient link adaptation mechanism as it may exist for example for unicast traffic.
However, even by using a very robust physical mode, some packet losses may occur because of radio channel variation and shadowing effect.
Such packet losses are particularly common with small hand devices such as smartphones or touch panels for example.
The packet losses are detrimental to the quality of the video displayed to the user, because they are translated into audio and video artefacts.
For those reasons, there is a need to introduce an efficient repetition mechanism for multicast video data flows to improve the audio/video quality experienced by the user.
In video coding, a codec arranges the pictures as a succession of intra- and inter-frames.
A Group Of Pictures, GOP, is a group of successive pictures within a coded video flow. A GOP can contain the following frame types:                I-frame, or intra-coded frame, which is a picture that is coded independently of all other pictures or frames. Each GOP starts with an I-frame;        P-frame, or predictive coded frame, contains motion-compensated difference information relative to previously decoded I or P frames;        B frame, or bi-predictive coded frame, contains motion-compensated difference information relative to previously and future decoded frames. B frames are generally not used as reference frames in order to avoid propagating errors.        
In a preferred implementation, the multicast data link can be transported over the IETF RTP (Real-time Transport Protocol) protocol. Even if HTTP (HyperText Transport Protocol) protocol is often used for video transmission, in particular for Internet streaming and Universal Plug and Play, UPnP, applications, this protocol is not adapted because it relies on TCP that is not compatible with multicasting. On the contrary, IPTV relies on IETF RTP over UDP protocol for data transport.
Data transport can be augmented by a control protocol such at Real-time Transport Control Protocol, RTCP, to allow monitoring of data delivery in a manner that is scalable to large multicast networks, and to provide minimal control and identification functionalities.
RTP and RTCP are standardized in IETF RFC 3550.
RTCP is also extended by IETF RFC 4585 in order to support messages for immediate feedback of the receivers, in particular negative acknowledgment, NACK, messages. In RFC 6642, a new message called “Third Party Loss Report” TPLR is introduced to mitigate the number of NACK messages induced when multiple receivers experience the same packet losses. A “feedback storm” troublesome effect may also generate collisions in wireless networks. There are therefore needs to design mechanism to avoid it.
The transmission of multimedia streams often relies on the RTP/RTCP protocols that are widely used, in particular for IPTV appliance.
Unlike HTTP, RTP/RTCP data streams are conveyed over UDP datagrams, and thus, delivery and packet ordering is not guaranteed. However, UDP datagrams can be multicast over wired or wireless networks.
In 802.11 (Wi-Fi) wireless networks, multicast flows can be transmitted in the same way as broadcasted flows: they are sent by an application provider AP without any acknowledgment policy (NoAck policy) and using a physical layer mode that is compatible with any receiver. As multicast data packets are not repeated, the physical layer mode is very conservative in order to ensure good reception for most of the receivers, even in case of bad radio conditions.
However, a conservative physical layer mode is not sufficient to deal with shadowing effects or fast radio channel changes.
Unlike Video-on-Demand (VoD) applications, live video streaming receivers do not support long buffering. Short buffers, coupled with the multiplicity of receivers, prevent from using retransmission schemes.
Audio/video decoders are resilient to data errors, but it leads to video frames dropping and/or video/audio artefacts.
Burst of packet losses due to the wireless network can have a huge impact on audio/video quality, especially when lost packets convey I-frame. Indeed, in that case, all the GOP will be impacted and numerous artefacts will be visible during several seconds.